The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to a method and apparatus for acquiring MR data with a segmented multi-shot radial fan beam encoding order. The invention further relates to reconstructing an image from the acquired MR data that is substantially free of eddy current induced artifacts.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
As is generally well-known, a number of MR imaging techniques has been developed to improve contrast between target anatomical features and background features. By improving the contrast between the target anatomical features and the background tissue, blood, etc., the diagnostic and probative value of the resulting image is also improved which facilitates more accurate, timely, and efficient diagnosis by health care providers. One such imaging technique that is widely used in cardiac applications is fully balanced steady state coherent imaging. This imaging technique (also known as b-SSFP, FIESTA, true-FISP, or b-FFE) provides high SNR per unit time and high CNR between transient signal of inflowing blood and steady-state signal of myocardial tissues, and, as such, has been exploited often for cardiac imaging. While effective for cardiac imaging, however, fully balanced steady state coherent imaging has not heretofore been applied to other clinical imaging applications. This is because the T2/T1 contrast typically achieved with such a technique does not provide clinically adequate soft tissues contrast. That is, soft tissue typically has low T2/T1 values and, as a result, conventional fully balanced steady-state coherent imaging techniques are ineffective.
While not applied with balanced steady-state coherent imaging techniques, such as b-SSFP, FIESTA, magnetization preparation techniques have been developed to improve soft tissues contrast. These magnetization preparation techniques include IR-prepared, T2-prepared, and FATSAT and they have been successfully employed with FSE, GRE, SGPR, etc. data acquisition paradigms. In order to maximize the magnetization prepared signal contrast during the acquisition of the center of k-space, a segmented centric phase ordering is generally necessary. Centric order b-SSFP acquisition, however, is sensitive to eddy current induced artifacts due to large phase encoding jumps between repetition times (TR). Moreover, these eddy current induced artifacts become more pronounced with a segmented centric phase ordering.
It would therefore be desirable to have a system and method capable of magnetization prepared contrast for fully balanced coherent imaging that reduces the change between successive phase encoding steps of a segmented acquisition to reduce the effect of eddy currents and thereby enhance the clinical feasibility of fully balanced coherent imaging to non-cardiac acquisitions.